coros.js

function producer(){
  return function*(){
    return yield io.read();
  }; 
}

function prepend(str, fn){
  return function*(){
    return str + yield fn();
  };
}

function uppercase(fn){
  return function*(){
    return (yield fn()).toUpperCase();
  };
}

while (true) {
  io.write(yield prepend('>>> ', uppercase(producer())));
}